CHEM 391 CATALYSIS LABORATORY MODULE

 

CHEM 391 ADVANCED MODULE IN CATALYSIS Credits 1.00 Spring 01

Preparation and study of a homogeneous catalytic system.

Taught in the second half of the semester.

 

Prereq-CHEM 351 Enr: 3 Max: 10 BH Rm. 230 01:00 PM - 5:00 PM

Instructor: Andrew R. Barron

 

• Syllabus
• Schedule
• Laboratory Notebooks
• Experiments

Instructor:	Andrew R. Barron
Office:	Butcher Hall 410, Ext. 5610
Email Address:	arb@rice.edu
Office Hours by Appointment
Teaching Assistants:	Chris Edwards, BH410, ext. 3456, chrised@rice.edu

What is a catalysis? The Oxford English Dictionary defines the "acceleration of a chemical or biochemical reaction by a catalyst" and a catalyst as "a substance that, without itself undergoing any permanent chemical change, increases the rate of a reaction".

Why are catalysts and catalysis important? Since the turn of the last century the chemical industry, in all its various forms, has become the cornerstone of the industrial nations. There is not another industry on the planet that has such an impact on our lives - in most cases without our knowledge. The PC would not exist if it wasn't for the chemicals that are used in the manufacture of the chips or the polymers that make the box. The modern automobile would not be possible if it wasn't for lubricants, coolants, adhesives, materials, fuel that are all produced by the chemical industry. Although the public believes it is the doctor that cures the patient, it in fact the chemical industries ability to make drugs with specific biological functions. Finally, our ability to feed the growing population of the world relies on the chemical industry to produce fertilizers and insecticides.

The conversion of one readily available or low cost chemical to another of greater economic value is the general goal of the chemical industry. However, there are many chemical reactions that will not occur under normal conditions or that require extreme conditions. Although a catalyst cannot alter the thermodynamic possibility of the reaction, it can speed up the rate of a reaction. Catalysts also provide alternative, lower-energy pathway to products; making it easier for a system to go from some initial to some final state. For example, the reaction of unsaturated hydrocarbons, olefins, with hydrogen will not occur under commercially feasible conditions, however, addition of a rhodium catalyst allows for the reaction to occur at room temperature.

Although catalysts are known for many chemical transformations, industry is always looking for improved systems. In recent years there has been much effort in developing "environmentally benign" chemical processes. The main thrust of this has been the development of more efficient catalysts that require a lower concentration of the catalyst, require lower temperatures and are more specific and selective. A decrease in catalyst concentration lowers and higher specificity and selectivity result in lower effluents and waste, while lower temperatures improved energy efficiency. Catalysts may also be used in the conversion of waste to environmentally benign or more importantly useful products. Lastly, not only is the identity of the product important, but in many cases (especially in agrochemical and pharmaceuticals) the chirality is important. Nature produces optically pure compounds, while man, in general, produces mixtures of optical isomers. The ability to produce only a single isomer of a biologically significant chemicals is therefore desirable.

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Grading:

The course grade is based upon the performance in the laboratory. This includes attendance (10%), preparation (10%), actual experimental skills developed (10%), results (20%) and the laboratory notebook (50%). You will work individually (except where indicated), thus you should each keep a completely independent notebook. In addition, I will rely heavily on my own observations of your work in order to assess your progress in the course. Your lab notebooks will be due by 5 PM, Friday, April 30, 2001; late notebooks will be penalized 5% per weekday late. It is recommended that you keep your lab notebook in your lab desk drawer to prevent loss.

Safety:

You must at all times were safety glasses and you must follow reasonable safety precautions when working in the lab!!! Failure to abide by the safety regulations will get you kicked out of the lab. This is an extremely serious issue with dire consequences to health and safety of yourself and your colleagues. Wearing a lab coat or apron is recommended but not required. Specific safety precautions concerning the use of hazardous chemicals and operation of scientific equipment will be indicated whether by myself or by one of the teaching assistants. These procedures must be followed. If you have any doubts about the safety of something you are about to do, ask first!!! We will be most happy to answer your questions. Finally, and very importantly, at least the T.A. or myself must be physically present at all times for you to work in the lab

If you have a documented disability that will impact your work in this class, please contact me to discuss your needs. Additionally, you will need to register with the Disability Support Services Office in the Ley Student Center.

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Catalysis Laboratory Module

 

Laboratory Notebooks

 

Your laboratory notebook should be a clear and accurate record of your experiments and calculations as they happen. Thus, when you are done with an experiment and all of the chemicals used have been washed away, and all the apparati taken down, the notebook will serve as an infallible "memory" of what happened and how. This is especially true for thesis work where some time may elapse before results are written up for the thesis or publication. Industrial standards for keeping notebooks are rigorous, due to patent and other legal concerns, so that all entries may need to be witnessed, dated and signed. The following guidelines will be used in this course.

 

1. Use a durable, sewn notebook, preferably cloth bound.

 

2. Always record data in permanent ink.

 

3. Do not record any data on scrap paper. Scrap paper used for this purpose will be taken up, and your data will be lost. Record all data directly into your notebook and date. Quantities including the raw measurements should be included.

 

4. Cross through mistakes in such a manner so that they may still be read.

 

5. Include your name, course title, semester date, college affiliation, and phone number on the title page in case your notebook is lost.

 

6. Number all pages in ink if this is not already done.

 

7. Include an updated table of contents at the beginning of the notebook. Leave 2 to 3 pages at the beginning of the book for this purpose.

 

8. Start a new page for each experiment.

 

9. The usual rules regarding significant figures should be observed. Never report more significant figures than warranted by the data.

 

10. Adopt a numbering scheme based on the date or notebook page for all samples and spectra. These numbers should appear on the sample/spectrum and when the sample/spectrum is mentioned in the notebook. ARB 101 A means the first sample on page 101 of Andrew R. Barron's notebook. Also on the spectrum should be written the composition of the sample and the sample number.

 

11. A compressed scan, photocopy, or sketch of the spectrum placed directly in the notebook is a useful aid when the data are being reviewed.

 

12. The origin of solvents and reagents should be clearly stated.

 

13. Make it clear what apparatus was used. Whenever an unusual piece of equipment is first used, a sketch of it should be included.

 

14. If physical data are used, or if spectral comparisons are made with the literature, the reference should be recorded.

 

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Lab Write Ups for Your Notebook:

 

If you wish to type your report. Print it with 1.5" margins and attach (with Scotch tape or similar) to your lab book.

 

What do I want for the lab write up? The compiled report should be in the format listed in the procedures below. It should be concise.

 

In the experimental section, the most important thing is to note details about things changed from the written procedure. You do not need to repeat the detailed experimental from the lab handout.

 

The results section is very important and should detail what happened (color changes, gas evolution, heat production, etc.). It should also list the analytical results (IR spectra, NMR spectra, melting points, etc.). It will probably be the longest section.

 

The discussion section is your chance to shine. Here you should describe problems you had with the synthesis, if any, and ways the experiment might be done better. You get to interpret the spectroscopic and spectrometric data. Concentrate on the highlights of the techniques as we discussed them in class. What are possible sources of error? Of low yields? Of impure materials? What did you learn in general? Feel free to discuss the experiments with the other lab groups, but write your own discussion in your own words.

 

The following lists specific sections to include in your write-up. Start each write-up on a new page. In general the write-up should follow the general format of the major chemistry journals - see attached.

 

1. Date. Be sure and date all entries in your notebook.

 

2. Title. The title should be as brief and as informative as possible. Titles should also be included for all drawings, charts, or graphs (these are often called "Legends").

 

3. Introduction. The introduction should include the purpose of the experiment, descriptions of any chemical or physical processes expected to occur, and any known literature values or other data pertinent to the experiment. For chemical processes, include all pertinent balanced chemical equations.

 

4. Procedure. Reference the source of the experimental procedure you are following. Explicitly note any alterations you make in the procedure, and any differences in the outcome. Securely attach any pertinent handouts directly to the notebook page.

 

5. Observations. Include all numeric data obtained during the experiment. Label all numeric data clearly. Also include detailed descriptions of all pertinent physical or chemical processes that you observe.

 

6. Results. Label all numeric results clearly. Include, when appropriate, any final values obtained, standard deviations of measurements, and percent errors, when a theoretical result is available.

 

7. Conclusion. Did the experiment work? List possible sources of error, and explain whether they would lower or raise numeric results, if possible. If you have suggestions for improving the experimental procedure, you should include them as well.

 

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NOTE: Lab notebooks are due by 5 PM, Friday, May 4th. Late notebooks will be penalized 5% per weekday late.

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Experiment 1: Recystallization of triphenylphosphine

Triphenylphosphine [P(C₆H₅)₃, I] oxidizes on storage to triphenylphosphineoxide [O=P(C₆H₅)₃, II], and must therefore be purified before use in Experiment 2.

 Set aside a small sample (ca. 0.5 g) of the crude P(C6H5)3 you are provided with in a screw top sample jar (this will be used to determine the efficiency of your recrystallization). Label the sample with your name and its identity so as not to get it confused.

Place about 10 g of P(C₆H₅)₃ into a conical flask and add 10 - 15 mL of ethanol (95%). All of the P(C₆H₅)₃ will not dissolve so don't panic. Warm the flask on a hot plate in a fume hood. CAUTION. Ethanol is flammable so this must be done in a hood. If you do not there is a danger of fire. Once all the P(C₆H₅)₃ has dissolved allow the flask to cool slowly to room temperature. (If all the P(C₆H₅)₃ does not dissolve, you can add another 10 mL of ethanol and re-heat. If there is still a precipitate ask a demonstrator before proceeding). After the flask has cooled to room temperature, filter the white solid through filter paper in a conical funnel.

(a) Set aside a small sample (ca. 0.5 g) of the "pure" P(C₆H₅)₃ in a screw top jar. Label the jar with your name and the sample identity. Place the remainder of your "pure" P(C₆H₅)₃ in a large screw top sample jar. This will be used for Experiment 2.

(b) In order to check the purity of your P(C₆H₅)₃, you will need to make-up two NMR samples. To do this get two NMR tubes and their tops. Make sure they are clean! Place a small amount of each of your P(C₆H₅)₃ samples in a NMR tube. The crude in one tube and the "pure" in the other! Add ca. 1 mL of CDCl₃ to each tube. CAUTION: CDCl₃ is very expensive - you drop it you buy it! Label each sample and submit to a demonstrator for NMR analysis.

(c) Record the relative % of P(C₆H₅)₃ and O=P(C₆H₅)₃ in each sample based one the 31P NMR spectra.

Q. 1. What was the % of O=P(C₆H₅)₃ in the commercial sample?

Q.2. What was the % of O=P(C₆H₅)₃ in your "pure sample"?

Q.3. Based on these results what can you say about the relative solubility of P(C₆H₅)₃ and O=P(C₆H₅)₃ in ethanol?

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Experiment 2. Synthesis of tris(triphenylphosphine)chlororhodium(I).

Commercial rhodium(III) chloride trihydrate usually corresponds closely to RhCl₃·3H₂O but small divergences from this stoichiometry are not significant in this preparation. The yield is calculated from the Rh content.

Rhodium(III) chloride trihydrate (0.5 g.) and 3 g. of triphenylphosphine (see Experiment 1) are placed into a flask fitted with gas inlet tube, reflux condenser, and gas exit bubbler. The flask is flushed with nitrogen to remove all the air. Ethanol (95%) is degassed in a syringe (see ARB) and added to the flask. The solution is refluxed for about 2 hours, and the crystalline product which forms is collected from the hot solution on sintered-glass filter. The product is washed with small portions of 10 mL of anhydrous diethylether (Et₂O). The final crystalline product is deep red in color.

1. Determine the yield by weighing the product.
2. Store your sample in a screw-top jar. Make sure you label the jar with your name and the identity of the contents.
3. An isomeric species which is orange is obtained if the total volume of ethanol used is 200 mL or less and the solution is refluxed for a period of about 5 minutes. This substance often contains small amounts of the red product and, on continued refluxing, the orange crystals are slowly converted to the red form. The orange product is not very active as a catalyst, so take care.
4. Prepare a sample of RhCl[P(C₆H₅)₃]₃, also known as Wilkinson's catalyst, for NMR analysis.

Q.1. What is the ³¹P NMR shift of the phosphines in RhCl[P(C₆H₅)₃]₃? How does this compare to the chemical shifts of "free" P(C₆H₅)₃ and O=P(C6H5)3? What does this tell you about the electronic environment of the P atom in each of these species? Can you explain the shift in RhCl[P(C₆H₅)₃]₃ by the type of bonding between Rh and P?

Q.2. At which University was Wilkinson a Professor when he made this compound? He was the "Sir Edward Franklin Chair of Chemistry", who was Sir Edward Franklin and what area of chemistry was he famous? What award did Wilkinson win, in part, for the discovery of this compound?

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Experiment 3: Hydrogenation of 1-Pentene

Wilkinson's catalyst is an active catalyst for the hydrogenation of olefins. As an example you are going to hydrogenate, 1-pentene (I).

Place an accurately weighed sample of Wilkinson's catalyst (ca. 0.3 g) and a magnetic "flee" into a thick glass bottle. Place a suba-seal onto the top and wire it on. Then using a needle as a gas inlet, add degassed toluene (30 mL). Make sure that the Wilkinson's catalyst dissolved. If it doesn't add some more solvent, but record the volume you add accurately. Add 1-pentene (6.0 mL) to your sample using a syringe. Remove a small sample (ca. 1 mL) from the reaction flask (using a syringe) and place in a small sample vial.

Take your glass reaction vessel over to the hydrogen line. Insert the hydrogen inlet needle and adjust the flow rate (see ARB for value) then start your stop watch (a normal watch is fine). make sure your flask has an outlet attached.

Record the time and start the stirrer. You have to make sure the solution has a large vortex to make sure hydrogen uptake occurs. Remove a (1 mL) sample after 2 minutes and place into a sample vial. Remember to label the vials. Repeat this process after 4, 6, 8, 10, 15, and 20 minutes. Label all samples. Remove the hydrogen flow and store your reaction flask.

Take all your samples for GC analysis.

(a) Calculate the ratio of olefin to alkane for each of your samples.

(b) Plot a graph of moles of alkane versus time.

(c) Assuming pseudo 1st order kinetics determine the rate constant for your experiment.

Q. 1. What is the real rate equation for the Wilkinson's catalyst olefin hydrogenation?

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Experiment 4: Effect of Excess PPh₃.

Repeat experiment 3, but add a weighed amount of PPh₃ to the reaction mixture prior to bubbling hydrogen through.

(a) Calculate the ratio of olefin to alkane for each of your samples.

(b) Plot a graph of moles of alkane versus time.

(c) Assuming pseudo 1st order kinetics determine the rate constant for your experiment.

Q. 1. What is the effect of adding PPh₃ to the rate of the reaction?

Q. 2. Which step of the catalytic cycle would the excess PPh₃ effect?

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Experiment 5: Handling Pyrophoric Compounds.

Place an accurately weighed sample of your phenol (ca. 2 mmol) and a magnetic "flee" into a Schlenk flask. Place a suba-seal onto the top. The Schlenk is evacuated and flushed with nitrogen. Then add degassed hexane (ca. 20 mL). Cool the reaction to -78 °C using a dry ice/acetone bath. Using a syringe add an equimolar amount of Al(^tBu)₃ from the standard solution (You will have to calculate exactly how much of the solution to add. Make sure you have your numbers checked).

CAUTION: The reaction of Al(^tBu)₃ with phenol gives off butane gas and is highly exothermic. This is why the reaction is performed at -78 °C.

Once the Al(^tBu)₃ is added stir the reaction while allowing it to warm to room temperature. Continue stirring for at least 30 mins. Remove all the volatiles under vacuum. You should now have a white solid. In order to make up the NMR sample you will need to use the dry-box in the Barron Group. See ARB to arrange this. Weigh all of your sample to obtain the yield of the crude product. Make up a sample for NMR spectroscopy and Mass Spectrometry. retain the remainder of your sample in the Schlenk. Recrystallize from hexane to obtain pure compound, see ARB.

Once you have pure compound determine the melting point to confirm purity.

(a) Calculate the yield of your product.

(b) Assign all the peaks in the ¹H and ¹³C NMR spectra for your product. You may need to re-run the NMR on your purified sample.

(c) Assign the mass spectrum of your material.

Q. 1. What is the structure of your product? See, J. Chem. Cryst., 1996, 26, 293 for the structural characterization of a similar compound.

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Experiment 6: Ziegler-Natta Polymerization of Ethylene.

In the dry box accurately weigh out ca. 20 mg of TiCl₃. Place in a glass bottle. Put a cap on the bottle and crimp in place. Bring the sample out of the dry box.

Add ca. 20 mL of degassed hexane to the TiCl₃ via a syringe. Then add an accurately measured amount of AlEt₃. CAUTION: AlEt₃ is pyrophoric. DO NOT expose to air. You should add approximately 200 molar equivalents Al per Ti. Allow the mixture to react for 20 minutes.

Bubble ethylene through the suspension for a measured amount of time. Polyethylene should form rapidly in an exothermic manner. Once ethylene addition is stopped, quench the reaction by cooling to -78 °C and adding EtOH (slowly).

After warming to room temperature the flask can be opened and the polymer removed and washed with hexane.

(a) Determine the polymer yield per g Ti per hour.

(b) Determine the mp and Dmp by TGA.

(c) Make up a sample for solid state ¹³C CPMAS NMR, see ARB.

Q. 1. What is the accepted mechanism by which this catalyst polymerizes?

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Experiment 7: Catalyzed H/D Exchange in Aromatic Compounds.

The samples will need to be prepared inside a dry box. Into a small glass bottle place HgCl₂ (ca. 5 mg) and two molar equivalents of AlCl₃ (ca. 4.9 mg). Add 1 mL of degassed toluene, and place a screw cap on the bottle. Wrap the bottle in aluminum foil and shake until all the solids dissolve. Remove the cap, and add 1 mL of C₆D₆ and shake well (remember to put the cap back on). Bring the bottle out of the dry box and leave to react for a set amount of time. Each student should choose a different time.

Quench the reaction by exposure to air and with the addition of a few drops of water. Shake well. Remove a sample of the organic layer for MS analysis.

(a) Calculate the relative amounts of each isomer of C₆H₅-_xD_xMe.

(b) Calculate how many H/D exchange reactions have occurred in total.

(c) Calculate how many H/D exchange reactions occurred per Hg atom per minute.

Q. 1. What other catalysts are used to promote H/D exchange?

For more information on this system, see Angew. Chem. Int. Ed., 2000, 39, 4117.

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